1. Field of the Invention
The present invention is directed to a system and method for extending the operating range and/or increasing the bandwidth of a communication link. The invention is applicable to the transmission of information over any type of xe2x80x9cband limitedxe2x80x9d media, including metallic twisted wire pairs, coaxial cables, fiber optic cables, and radio frequency (xe2x80x9cRFxe2x80x9d) paths.
2. Description of Related Art
Over the last several years, the demand for high-speed data communication services has exploded. Most homes and small businesses, for example, have at least one personal computer (xe2x80x9cPCxe2x80x9d) with a dial-in modem that can be used to connect with an Internet service provider (xe2x80x9cISPxe2x80x9d) via the public switched telephone network (xe2x80x9cPSTNxe2x80x9d). These dial-in modems generally operate at 14.4 kb/s, 28.8 kb/s, 33.6 kb/s, or even 56 kb/s. As such, dial-in Internet connections are capable of providing basic Internet services, including Web access, electronic mail, home banking, electronic commerce and the like.
There are a number of problems, however, associated with the provision of basic Internet services via the PSTN. For example, dial-in modems cannot be used to provide high-speed Internet or other broadband services that require a communication link of 1-2 Mb/s or greater, such as broadcast video, movies-on-demand, distance learning and the like. Also, because Internet data packets are transmitted between the dial-in modems and the ISPs through a voice switch located at the telephone company central office, a significant number of Internet connections (which typically have long holding times) can cause blockage to occur in the switch. Under extreme conditions, essential calls (e.g. 911 calls) may experience dial tone delays. In addition, high volumes of Internet traffic can overload the trunking network that connects the voice switch to the ISPs.
In an effort to overcome the problems associated with dial-in Internet connections, various x-type digital subscriber line (xe2x80x9cxDSLxe2x80x9d) technologies have been developed, the most prevalent of which is asymmetric digital subscriber line (xe2x80x9cADSLxe2x80x9d) technology. Referring to FIG. 1, a typical ADSL system includes a plurality of remote ADSL transceiver units (xe2x80x9cATU-Rxe2x80x9d) a located at various homes and businesses, and a plurality of complimentary central office ADSL transceiver units (xe2x80x9cATU-Cxe2x80x9d) b located at the telephone company central office. Each ATU-R a communicates with each ATU-C b over a single twisted wire pair c. ATU-R a and ATU-C b can typically support data rates of up to 640 kb/s upstream (i.e. from ATU-R a to ATU-C b) and up to 6 Mb/s downstream (i.e. from ATU-C b to ATU-R a), hence the term xe2x80x9casymmetric.xe2x80x9d
At each of the various homes and businesses, ATU-R a can be linked to a PC d for the provision of high-speed Internet services, to a TV set top box e for the provision of video services (e.g. movies-on-demand), and/or to any other type of broadband device. In addition, most ADSL systems support plain old telephone service (xe2x80x9cPOTSxe2x80x9d). In those systems, a POTS splitter f is provided to filter out the 4 kHz analog voice signal and route it to an existing analog telephone g. Alternatively, POTS splitter f may be integrated within ATU-R a.
At the telephone company central office, each ATU-C b is connected to a digital subscriber line access module (xe2x80x9cDSLAMxe2x80x9d) h. DSLAM h concentrates and/or switches the various data signals and routes them to their appropriate destination, such as an ATM switch i, an IP router j, or other broadband devices k. All of these devices are in turn connected to a broadband network, thereby relieving the congestion problems associated with the transmission of data signals over the PSTN. If the ADSL system supports POTS, a POTS splitter l is provided to filter out the 4 kHz analog voice signal and route it to a voice switch m of the PSTN. Alternatively, POTS splitter l may be integrated within ATU-C b.
Although various line coding techniques may be used for the transmission of information between ATU-R a and ATU-C b, the most common is discrete multi-tone (xe2x80x9cDMTxe2x80x9d) line coding as adopted by the ANSI T1.413 (1995) standard.
As shown in FIG. 2, DMT line coding is used to divide the information transmitted over twisted wire pair c between 256 subcarriers, each of which occupies 4.3125 kHz for a total bandwidth of 1.104 MHz. While most of the subcarriers are used to carry voice and data signals, some are used for network management and performance measurement functions (e.g. subcarrier #64 at 276 kHz is reserved for a downstream pilot signal) and others are not used at all (e.g. those subcarriers affected by a bridged tap, radio-frequency noise, or impulse noise).
As shown in FIG. 3, the 1.104 MHz frequency spectrum is divided into four frequency bandsxe2x80x94a voice band, a guard band, an upstream band, and a downstream band. The voice band, which occupies the lower portion of the frequency spectrum between 0 Hz and 4.3125 kHz (i.e. subcarrier #1), is used to carry a 4 kHz analog voice signal. The guard band, which occupies the next portion of the frequency spectrum between 4.3125 kHz and 25.875 kHz (i.e. subcarrier #""s 2-6), is used to separate the voice band from the upstream and downstream bands. The upstream band, which occupies the next portion of the frequency spectrum between 25.875 kHz and 138 kHz (i.e. subcarrier #""s 7-32), is used to carry data signals from ATU-R a to ATU-C b. The downstream band, which occupies the upper portion of the frequency spectrum between 25.875 kHz and 1.104 MHz (i.e. subcarrier #""s 7-256), is used to carry data signals from ATU-C b to ATU-R a. Thus, with DMT line coding, ATU-R a and ATU-C b are capable of providing both POTS and high-speed Internet and other broadband services over the same twisted wire pair c.
While ADSL technology overcomes the problems associated with dial-in Internet connections, it has its own set of attendant problems. For example, current FCC regulations (i.e. FCC Part 68) limit the maximum power that can be applied to twisted wire pair c so as to control the transmission of small broadcast signals that can interfere with surrounding signals. Because of these power constraints, the distance between ATU-C b and ATU-R a is limited due to the attenuation of the signal at the far end and resulting cross-talk from other twisted wire pairs. This distance will vary according to the data rate being offered, the gauge and generation of twisted wire pair c, the number of bridged taps and other factors. As a result, homes and businesses that are located more than a certain distance from the telephone company central office are not able to receive high speed Internet and other broadband services.
One attempt to solve this problem has been to regenerate the attenuated signal by strategically placing a mid-span DSL repeater on twisted wire pair c between ATU-R a and ATU-C b. Although the cost of the DSL repeater itself is relatively small, the cost of the environmentally-hardened case surrounding the repeater and the labor required to splice the repeater into the twisted wire pair can be prohibitive. In addition to cost issues, there are other problems with powering the DSL repeater and with simply finding a mid-span location to mount the DSL repeater.
Another problem associated with ADSL technology is that ATU-R a and ATU-C b can only support data rates of up to 6 Mb/s downstream, even at the shortest distances. As such, ATU-R a and ATU-C b cannot be used to provide very high speed Internet or other broadband services that require a communication link of 6 Mb/s or greater, such as high definition television (xe2x80x9cHDTVxe2x80x9d), high-performance business applications and the like.
Thus, while dial-in Internet connections and ADSL technology have satisfied some of the demand for high speed data communication services, a need remains for a system that is capable of transmitting high speed Internet and other broadband services to homes and businesses that are located at greater distances from the telephone company central office. There is also a need for a system that is capable of providing very high speed Internet and other broadband services that require a communication link of 6 Mb/s or greater over the existing twisted wire pair infrastructure.
The present invention is directed to a system and method for extending the operating range and/or increasing the bandwidth of a communication link. In broad terms, the communication link may consist of any two communication devices that are connected by two or more separate xe2x80x9cband limitedxe2x80x9d communication paths for the transmission of information therebetween. In operation, the communication devices use a xe2x80x9cfrequency split schemexe2x80x9d to divide the information transmitted between the devices into two or more signals, each of which is transmitted over one of the communication paths. A xe2x80x9cfrequency foldback schemexe2x80x9d may also be used to shift the information from the higher frequencies to the lower frequencies of each respective signal, thereby taking advantage of the fact that more information can be carried in the lower frequencies. By using these schemes, it is possible to extend the operating range of the communication link and/or transmit a greater amount of information over the communication paths to thereby increase the bandwidth of the communication link.
In a preferred embodiment of the present invention, the communication link comprises an ADSL system in which a remote ADSL transceiver unit (xe2x80x9cATU-Rxe2x80x9d) communicates with a central office ADSL transceiver unit (xe2x80x9cATU-Cxe2x80x9d) over two or more twisted wire pairs. In this embodiment, the ATU-R and ATU-C are designed to use the xe2x80x9cfrequency split schemexe2x80x9d and xe2x80x9cfrequency foldback schemexe2x80x9d for the purpose of extending the operating range of the ADSL system, while at the same time transmitting a greater amount of information therebetween. Alternatively, the ATU-R and ATU-C could be designed to use the xe2x80x9cfrequency split schemexe2x80x9d in such a manner as to increase the bandwidth of the ADSL system to an even greater extent.
In a first alternative embodiment of the present invention, the communication link comprises a hybrid fiber/coaxial cable distribution system. This system includes a head end that is connected to a plurality of nodes via a high-speed fiber optic backbone. Each of these nodes is in turn connected to a plurality of subscriber homes via two or more coaxial cables. At each subscriber home, a splitter is provided to divide the signal transmitted over the coaxial cables between a TV set top box for the provision of cable television programming and a cable modem for the provision of high-speed Internet services. In this embodiment, the head end and cable modems are designed to use the xe2x80x9cfrequency split schemexe2x80x9d and xe2x80x9cfrequency foldback schemexe2x80x9d for the purpose of extending the operating range of the distribution system, while at the same time increasing the bandwidth of the system to thereby serve a greater number of homes off the coaxial cable line.
In a second alternative embodiment of the present invention, the communication link comprises a fiber optic system in which a modulator communicates with a demodulator via two or more fiber optic cables. The fiber optic cables may consist of either multi-mode or single-mode fiber optic cables, depending upon the application. Alternatively, the xe2x80x9chard-wiredxe2x80x9d fiber optic cables could be replaced by a switched optical network, whereby wavelength-division multiplexing is used to transmit multiple optical signals having different wavelengths over the same fiber optic cables. In this embodiment, the modulator and demodulator are designed to use the xe2x80x9cfrequency split schemexe2x80x9d and xe2x80x9cfrequency foldback schemexe2x80x9d for the purpose of extending the operating range of the fiber optic system, while at the same time increasing the bandwidth of the fiber optic system to thereby enable the transmission of higher data rates.
In a third alternative embodiment of the present invention, the communication link comprises a radio system in which a modulator communicates with a demodulator via two or more RF paths. The RF paths may consist of any fixed xe2x80x9cband-limitedxe2x80x9d path, such as a 30 MHz path used for the transmission of an HDTV signal, a 6 MHz path used for the transmission of a television signal, a 15 kHz path used for the transmission of an FM radio signal, or a 5 kHz path used for the transmission of an AM radio signal. Alternatively, the RF paths may consist of CDMA radio channels, wherein each channel consists of a particular frequency/pseudo-random number sequence. In this embodiment, the modulator and demodulator are designed to use the xe2x80x9cfrequency split schemexe2x80x9d and/or xe2x80x9cfrequency foldback schemexe2x80x9d for the purpose of extending the operating range and/or increasing the bandwidth of the radio system.